Avian myeloblastosis virus (AMV) is defective in reproductive capacity and thus requires a helper such as myeloblastosis associated virus (MAV). The genome of AMV has undergone a sequence substitution in the region normally coding for the viral envelope protein which has been replaced by cellular sequences responsible for leukemogenesis. Comparison of the nucleotide sequences of AMV and MAV indicates that the recombination site occurs right after the sequences TTTTTGCAGGC, 80 nucleotides downstream from a KpnI site near the 3' end of the polymerase gene. This sequence matches well with the consensus sequence for the 3' RNA acceptor site. The preservation of the 3' splice signal in both AMV and MAV suggests that it may have been involved in the original recombination event, as well as being required as an acceptor site for LTR-promoted message synthesis. Furthermore, the amino acids predicted from the nucleotide sequences show that the carboxyl-terminus of MAV reverse transcriptase is different from that of AMV. We assume the MAV reverse transcriptase is a functional protein, because 60 of the 64 amino acids encoded from the KpnI site to the terminator are identical to those of Rous sarcoma virus (RSV). It is not clear yet whether AMV produces a functional polymerase with its cellular-derived carboxyl-terminus.